The continuous solution polymerization process is well known. Y. V. Kissin briefly discusses, in The Kirk-Othmer Encyclopedia of Chemical Technology, in an article titled “Polyethylene, Linear Low Density”, a solution polymerization process. In the solution process, solvent, monomer(s) and catalyst are continuously fed to a reactor. The reactor can be operated over a relatively wide range of temperatures and pressures; producing a single liquid phase containing the desired polymer. Downstream of the reactor, the single liquid phase is phase separated to recover the solvent, unreacted ethylene and α-olefins (if present) from the polymer. In the phase separation step, a first vapor/liquid (hereafter V/L) separator operating at lower pressure, relative to the reactor(s), generates: a gaseous overhead stream of solvent, monomers, hydrogen (if present), light-end impurities and possibly some low molecular weight oligomers or grease, and; a bottom stream of an ethylene polymer rich solution and deactivated catalyst.
The gaseous overhead stream produced in the first V/L separator is typically transported to a process unit that separates the components into chemically distinct fractions. Various processes are known to accomplish this separation; for example a distillation column or two or more distillation columns connected in series. Such distillation operations may also include a cryogenic distillation column for the separation of ethylene. The distilled products, e.g. solvent, comonomer(s) and ethylene can be stored in tanks or vessels prior to being transported to the upstream solution polymerization process. Engineers experienced in the art are familiar with the design of distillation columns to accomplish specific separations, e.g. Perry's Chemical Engineers' Handbook (8th Edition), D. W. Green and R. H. Perry, 2008 McGraw-Hill, Section 13, “Distillation”. The distillation operation is not particularly important to the success of this invention; however, this invention allows one to reduce the size and capacity of the distillation operation.
An earlier Canadian application (CA 2,809,718), that is related to this invention, discloses a process wherein a gaseous overhead stream from a first V/L separator is condensed and recycled to one or more upstream reactors in a more energy efficient manner; relative to passing this gaseous overhead stream to a distillation column.
The bottom stream produced in the first V/L separator may be transported to: i) a polymer recovery operation, or; ii) one or more additional V/L separators to remove additional solvent and optional comonomers. The subject matter of this application is focused on the latter, ii), as will be discussed in the invention summary below. Polymer recovery operations are not particularly important to the success of this invention. A typical polymer recovery operation includes a means for conveying the bottom stream, which is a viscous stream comprised essentially of molten ethylene polymer that contains a small amount of deactivated catalyst and residual solvent through a devolatilizing operation and ultimately through a pelletizer. Once pelletized, and optionally dried, the ethylene polymer is generally transported to a product silo. The means for conveying the bottom stream may include gravity, gear pumps, single screw extruders, twin screw extruders and sub-atmospheric pressure, vacuum extruders with vents that allow residual solvent or optional α-olefin comonomers to be removed.
The solution polymerization process is an energy intensive process. For example, relative to gas phase polymerization reactors, the solution polymerization reactor(s) run hotter, consume more steam and operate at higher pressures. A need exists to improve the energy efficiency of the continuous solution polymerization process. This invention describes embodiments of a continuous solution polymerization process that consumes less energy, relative to a base case solution polymerization process. Because less energy is consumed, manufacturing variable costs are reduced and the environment benefits, e.g., reduced greenhouse gas emissions. An added benefit of this invention is a reduction in the amount of capital required to build the continuous solution polymerization plant.